Polyolefin microporous films are broadly used as separation and permselective separation membranes, separator materials and the like for various types of substances, and broadly utilized in medical applications such as microfiltration membranes, electric material applications such as separators for fuel cells and separators for capacitors, and the like. Among these applications, the polyolefin microporous films are used especially suitably as separators for lithium ion batteries of mobile devices such as notebook personal computers, cell phones and digital cameras. In recent years, along with broadening of demands for industrial batteries, secondary batteries as housing backup power sources, and vehicular large-size batteries for electric automobiles, hybrid automobiles and the like, higher performance has been demanded also on separators.
Separators for batteries are required to have a function as a separator to separate a positive electrode and a negative electrode and permeate ions only, and additionally have a shutdown function (a function of clogging pores at a temperature lower than a thermal runaway temperature) to close pores due to melting of the pores when a large current flows to thereby prevent the cell reaction from runaway, and required to be thin membranes and have a high mechanical strength. Further in vehicular batteries, since the electrode area is enlarged for making the batteries of a high-output, and the proportion of the cost of their separators accounting for the whole cost of the batteries increases with the enlargement, the thickness reduction, the high productivity and the like of the separators are demanded.
Examples of methods for manufacturing a polyolefin microporous film include methods of Patent Literatures 1 to 4. Patent Literature 1 discloses a manufacture technology in which a raw film before being provided with pores is cold-stretched in MD direction and thereafter heat-stretched in MD direction and further heat-stretched in TD direction to thereby improve the air permeation with the dimensional stability being maintained. Patent Literature 2 discloses a method of using a kind of polyethylene having a specific molecular weight, or as required, concurrently using a polyethylene having a higher molecular weight, to thereby improve permeation and mechanical properties. Patent Literature 3 discloses a method of manufacturing a microporous film having the air permeation by using a composition having an elongation viscosity to a specific shearing viscosity. Patent Literature 4 discloses a method of decreasing the shutdown temperature by blending a high-density polyethylene and a polyethylene wax.